Matt Smith does a lot of running as the Doctor, and a large number of stunts, too. And he's kind of clumsy, they say.

...

So how the heck did he crack his head open AT HOME?

Maybe he was hammered?

The fellow playing Adric was hungover when shooting Castrovalva, and at the end when The Doc and the girls are looking at Tegan's landing....the reason we did not see Adric was because he was behind a tree, honking his guts out from the previous night's partying.